First, the King's Troop Royal Horse Artillery, wearing full dress uniform, rode their horses and gun carriages past Buckingham Palace to Hyde Park to stage a 41-gun Royal Salute at midday.

The Band of the Royal Artillery played as the 71 groomed horses pulled the six First World War-era 13-pounder field guns into position, before firing blank artillery rounds at precise 10-second intervals.

An hour later, the Honourable Artillery Company drove through the city in their liveried Pinzgauer vehicles to the Tower of London, firing a 62-gun salute across the Thames.

The bells of Westminster Abbey, where the Queen was married and crowned, also sounded at 1pm.

Later, th e Houses of Parliament, where MPs have paid tribute to the monarch, will be illuminated red, white and blue for the special royal anniversary through the night until dawn on Friday.

In the evening of her milestone anniversary, the Queen will light a beacon - the first in a chain of more than 1,000 across Britain and the world.

The Duke of Edinburgh, the Prince of Wales and the Duchess of Cornwall will accompany her.

Local authorities around the UK will host lighting events throughout the country to create a network of flames in celebration.

Two VIPs, both of whom celebrated their 90th birthday on the same day as The Queen (Thursday, 21 April), Albert Moore from East Belfast (left) and Austin Henderson from Stranmillis (right) assist Deputy Lord Mayor Alderman Guy Spence to light the beacon on the front lawn of City Hall. The Belfast tribute was part of a UK-wide beacon lighting programme marking the Royal birthday. Picture by Jonathan Porter/PressEye

Press Eye - Belfast - Northern Ireland - 21st April 2016
A mural on Rockview Street in the Village area of south Belfast on the day the Queen celebrates her 90th birthday. Picture by Jonathan Porter/PressEye

Pacemaker Press Belfast 21-04-2016: A number of events are being held in Northern Ireland to celebrate the 90th birthday of the Queen. A 21-gun salute took place at Hillsborough Castle and at other venues across the UK at noon.
Picture By: Arthur Allison.

Pacemaker Press Belfast 21-04-2016: A number of events are being held in Northern Ireland to celebrate the 90th birthday of the Queen. A 21-gun salute took place at Hillsborough Castle and at other venues across the UK at noon.
Picture By: Arthur Allison.

Pacemaker Press Belfast 21-04-2016: A number of events are being held in Northern Ireland to celebrate the 90th birthday of the Queen. A 21-gun salute took place at Hillsborough Castle and at other venues across the UK at noon.
Picture By: Arthur Allison.

Pacemaker Press Belfast 21-04-2016: A number of events are being held in Northern Ireland to celebrate the 90th birthday of the Queen. A 21-gun salute took place at Hillsborough Castle and at other venues across the UK at noon.
Picture By: Arthur Allison.

The beacons will be specially built gas-fuelled structures, traditional bonfires or braziers on top of a tall wooden post.

Members of the Army Cadet Force will be taking beacons to the top of the four highest peaks in the United Kingdom - Ben Nevis in Scotland, Mount Snowdon in Wales, Scafell Pike in England, and Slieve Donard in Northern Ireland.

Pageant master Bruno Peek, who was responsible for the Diamond Jubilee beacons, has been overseeing the plans.

Heir to the throne Charles has written a message to beacon lighters.

"It is a wonderful gesture which I know has deeply touched Her Majesty," he wrote.

"Beacons are to be lit on mountaintops, on beaches, in farmyards and on church towers, uniting us all in our heartfelt appreciation of the Queen's lifetime of service to the United Kingdom and other Realms, and to the Commonwealth."

There is a long and unbroken tradition of celebrating royal jubilees, weddings, coronations and birthdays in this way.

In Edinburgh, a firework display will be staged over the Royal Yacht Britannia.

Meanwhile, a chocolate sculpture of Buckingham Palace and a birthday message written on the hair of a corgi were among the more unusual ways people marked the occasion.

Chocolatiers at Cadbury World in Birmingham took four days to create the 132lb (60kg), 27.5in (70cm) tall edible model of the palace, complete with gates and chocolate soldiers standing guard.

And scientists at the University of Nottingham used a focused ion beam with nanoscale precision to inscribe the words "Happy 90th Birthday Your Majesty" on a single strand of hair from the Queen's favoured dog breed.